The instant invention relates to a circuit for compensating a notch filter attenuation at frequencies less than cut-off frequency thereof. More specifically, it concerns the application of such a circuit in the field of telephony to a Subscriber Line Interface Circuit (SLIC).
FIGS. 1 to 3 are to illustrate the problem that the instant invention intends to resolve.
FIG. 1 shows a line 10 wherein is interposed a resistor R and wherein current I.sub.in is supplied by a current source 11. An LC serial filter is connected across the line and a reference voltage node (currently the ground) to remove a definite frequency F.sub.0.
The response curve of an LC serial filter is schematically shown in FIG. 2. It can be seen that if attenuation is optimum, for frequency F.sub.0, it is far from negligible at frequencies remote from F.sub.0. For instance, if F.sub.0 is substantially equal to 15,000 Hz, attenuation will still be 0.1 dB at frequency F.sub.1 in the order of 3,500 Hz while the band width at 26 dB will be in the order of 250 Hz.
The drawbacks of such a filter are illustrated hereinafter as part of a particular embodiment.
FIG. 3 is a highly schematic representation of a Subscriber Line Interface Circuit (SLIC). Such circuits are conventionally arranged at the Central Office Exchange to establish connection with each subscriber line 20. In particular, they ensure that the Central Office Exchange has a given output impedance, for instance for French standards, a resistive impedance equal to 600 ohms in the 300-3,400 Hz frequency band. Such impedance accuracy is defined under various standards and characterized by the so-called return loss (RL) attenuation measurement. This measurement characterizes the difference between the desired impedance ZO and the real impedance Z by the relationship RL=20 log .vertline.(Z-ZO)/(Z+ZO).vertline.. This return loss RL is defined in France as having to be better than 20 dB.
Conventionally, an electronic subscriber interface is achieved by using one or more integrated circuits. In the example disclosed herein, there is a first integrated circuit 21 comprising more particularly balanced amplifiers 22 and 23 with gain G, driving the line, and means 24 for measuring line current i.sub.L and supplying a current signal proportional to i.sub.L, ki.sub.L. By means of a resistor R and a current mirror 25, this current is returned to a resistor R' which applies a feedback voltage to the input of amplifiers 22 and 23, said resistor being connected in parallel with capacitor C' designed to ensure loop stability. The elements shown in block 21 form, for example, part of the telephone integrated circuit marketed under reference TDB 7722 by the Company SGS Thomson and current mirror 25 forms part of another telephone integrated circuit marketed by SGS Thomson under reference TDB 7711.
Of course, means for entering the voice signal to be transmitted or for receiving the incident voice signal are provided in the integrated circuits at access terminals, not shown.
These various circuits produce highly satisfactory operation, i.e., matching attenuation greater than 20 dB in the 300-3,400 Hz frequency range.
In some cases, it is desirable to add to a telephone system a subscriber remote-charging system (telecharge). For this purpose, a high-frequency signal has to be transmitted on the line, for instance in the order of 15,000 Hz. However, because the impedance of the SLIC is not matched to these high frequencies, only a very weak signal will be sent on the line. Therefore, in the prior art, the addition of a filter was considered for instance in block 26 shown in dotted lines in FIG. 3. This filter lowers the impedance of the SLIC at the frequency of the charging signal.
However, if a single filter as shown in FIG. 1 is used, as is generally the case, the drawback presented previously is still found, i.e., the upper part of the useful frequency range (e.g. 300-3,400 Hz) has a signal which is unduly attenuated so that the SLIC no longer assumes its impedance matching function satisfactorily.
To avoid such drawbacks which result from the introduction by a notch filter of an attenuation or phase shift which is a hindrance when far from its cut-off frequencies, various solutions have been proposed in the prior art:
using values of C and L in such a way as to have a narrower filter. This results in a longer filter response time which constitutes another drawback ;
producing a larger scale filter by using a more complex network of inductances and capacitors. Yet, another difficulty is involved herewith. Indeed, when filters are to be produced in relation with integrated circuits, inductance L is advantageously produced by means of a capacitor and an electronic circuit referred to as a gyrator. Indeed, a gyrator is practical to construct if one of the inductance terminals is connected to the ground. Otherwise, in the case of a more complex filter comprising inductances which are not connected to the ground, a difficult problem arises in the production of floating inductances, leading to the use of a great number of operational amplifiers, i.e., a large size integrated circuit.
Therefore, one object of the instant invention is to resolve the problem submitted without sustaining the drawbacks of the devices embodied in the prior art. More particularly, the instant invention is aimed at improving the attenuation and phase shift of a notch filter at frequencies smaller than the cut-off frequency F.sub.0 but without modifying the filtration around frequency F.sub.0 in a low-cost manner by the use of a simple filter, that filter being implementable as an integrated circuit.